ci: upload code coverage to Codecov

[malept]

Tracks test code coverage via Codecov.

TODO

• badge in README

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@42793b7). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #71   +/-   ##
=======================================
  Coverage        ?   75.00%           
=======================================
  Files           ?        1           
  Lines           ?       56           
  Branches        ?        0           
=======================================
  Hits            ?       42           
  Misses          ?       14           
  Partials        ?        0           

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